1. Field of the Invention
The present invention relates to a method of producing a fiber wavelength-conversion element with a core made of organic nonlinear optical crystal and a clad made of glass which is coated on the incident and outgoing end surfaces of the fiber with a macromolecular film. Optionally, particles and macromolecules may be packed into a recessed portion of an outgoing end surface.
2. Description of Related Art
Conventionally, laser light has been wavelength-converted into second harmonic waves thereof or the like by utilizing a non-linear optical material. A wavelength-conversion element for performing the wavelength conversion of the bulk crystal type has been described, for example, in A. Yarive, "Foundation of Optical Electronics", (translated by Kunio Ota and Takeshi Kamiya), MARUZEN Co. Ltd., pp. 200-204.
Recently, however, a fiber wavelength-conversion element has been used in which a material having large nonlinearity and having either small birefringence or none at all may be used and in which the phase matching between the fundamental wave and the second higher harmonic may be easily attained. This fiber wavelength-conversion element has a configuration in which a core of nonlinear optical material single crystal is formed in a clad of glass (for example, reference is made to Microoptics News of The Group of Microoptics, The Optical Society of Japan (The Japan Society of Applied Physics), Vol. 3, No. 2, pp. 28-32).
Further, a wavelength-conversion element having the same advantages as those of the fiber wavelength-conversion element is known in which as a core, an optical waveguide is formed of single crystal of a nonlinear optical material between two glass plates constituting a clad (for example, reference is made to Japanese Patent Unexamined Publication Nos. Sho. 63-15233 and Sho. 63-15234).
As the nonlinear optical material to be used for the core of the above fiber wavelength-conversion element, the use of a single crystal of an organic nonlinear optical material is recently proposed, although an inorganic nonlinear optical material such as KH.sub.2 PO.sub.4, lithium niobate (LiNbO.sub.3), or the like, has been used conventionally.
Organic nonlinear optical materials that can be used include, for example, 2-methyl-4-nitroaniline (MNA), metanitroaniline (mNA), 3-methyl-4-nitropyridine-1-oxide (POM), urea, N-(4-nitrophenyl)-(S)-prolinol (NPP), 2-{N-(4-nitrophenyl)-N-methylamino} acetonitrile (NPAN), 2-dimethylmethylamino-5-nitroacetamide (DAN), 2-N(.alpha.-methylbenzylamino)-5-nitropyridine (MBA-NP), or the like. These materials are described in "Nonlinear Optical Properties of Organic and Polymeric Materials", ACS SYMPOSIUM SERIES 233, edited by David J. Williams, American Chemical Society, 1983, "Organic Nonlinear Optical Material" supervised by Masao Kato and Hachiro Nakanishi, CMC Co., 1985, "Nonlinear Optical Properties of Organic Materials and Crystals", edited by D. S. Chemla and J. Zyss, Academic Press Inc., 1987, and "The Quality and Performance of The Organic Non-Linear Optical Material, 2-N(.alpha.-Methylbenzylamino)-5-Nitropyridine (MBA-NP)", Optical Communications, Vol. 63, No. 3, p. 223. Other organic non-linear optical materials which may be used are 3.5-dimethyl-1-(4-nitrophenyl)pyrazole (hereinafter, referred to as PRA), 3,5-dimethyl-1-(4-nitrophenyl)-1,2,4-triazole, 2-ethyl-1-1(4-nitrophenyl) imidazole, 1-(4-nitrophenyl) pyrrole, 2-dimethylamino-1, 5-nitroacetanilide, 3-methyl-4-nitropyridine-N-oxide, or the like, as disclosed in Japanese Patent Unexamined Publication No. Sho. 62-210432.
A single crystal of the above organic nonlinear optical material is essentially superior to that of an inorganic material in the points of (i) the large nonlinear polarizability, (ii) the improved light damage property, (iii) the high-speed response to electric field and the like. For example, the above MNA has wavelength conversion efficiency about 2000 times as high as that of LiNbO.sub.3.
In a fiber wavelength-conversion element using a single crystal of an organic nonlinear optical material as the core, the organic nonlinear optical crystal which is the core contacts with the atmosphere, such as surrounding air or the like, on the fiber end surfaces so that the quality of the core is sublimated or changed. Therefore, there has been a problem in that as time elapses, the wavelength conversion efficiency of the wavelength-conversion element and the incident coupling efficiency of a fundamental wave deteriorate remarkably and the occurrence of light loss increases.
As a method for solving the foregoing problem, Japanese Patent Unexamined Publication No. Hei. 2-79033 discloses a method of producing a wavelength-conversion element in which an organic non-linear optical material is dissolved in liquid resin to a saturated state and the solution is applied onto the end surfaces of a fiber. An organic nonlinear optical material is dissolved in liquid resin to a saturated state and the solution is applied onto the end surfaces of a fiber. An organic nonlinear optical material is however dissolved by about one weight percent even in liquid fluoresin having the lowest solubility.
In the foregoing method, therefore, if the temperature comes down by even 0.5.degree. C. from the saturation temperature of the liquid resin in which the organic nonlinear optical material is dissolved, the crystal forming the core may be dissolved. If the temperature comes up by 0.5.degree. C., on the contrary, the solution becomes supersaturated so that the organic nonlinear optical material deposits as crystal on the end surfaces of the core. As a result, it has been difficult to stably produce a light wavelength conversion element.
Further, Japanese Patent Unexamined Publication No. Hei. 2-79032 discloses a method of producing a fiber wavelength-conversion element in which the end surfaces of a fiber are coated with an aqueous solution of a vinyl acetate-acryl copolymer emulsion.
However, in this method, there has been a problem since the organic nonlinear optical material is somewhat dissolved in the aqueous solution, resulting in the reduction of the incident coupling efficiency of a fundamental wave. Further, this problem is remarkable in a fiber wave element having a core diameter (0.5-2 .mu.m) capable of obtaining a high efficiency of wavelength conversion.